Modelling the Physiological, Biogeochemical and Physical Processes Controlling the Interaction of Corals with Their Environment

Coral reef ecosystems are tightly interacting with their environment through physiological, biogeochemical, and physical processes. Hydrodynamic processes such as waves and currents not only control seawater chemistry by transporting material properties over the reef, but also determine the exchange rate of dissolved inorganic carbon, alkalinity and oxygen between the seawater and individual corals. These transports directly affect the physiological processes happening inside the corals. In return, coral calcification and photosynthesis provide a dynamic feedback to seawater chemistry. In order to quantify the state and variability of these complex interactions between the corals and the surrounding seawater, we developed a comprehensive 3D coupled hydrodynamic/biogeochemical/physiological model. This model builds on the Regional Oceanic Modelling System (ROMS), and was configured for the northern shore of Moorea Island at a resolution of 20m. Our simulations indicate that the spatiotemporal patterns of the chemical state of seawater and coral metabolism (photosynthesis, calcification etc.) are highly correlated. Among all potential factors, circulation, and especially wave-driven flow, is the main control of the spatial variability in seawater chemistry. Temporally, the strong diurnal cycle of photosynthesis is the pacemaker, propagating down to the calcifying fluid in corals, as well as how coral's inter-cellular cycles are reflected in the chemical state of the seawater. For instance, corals create up to 30% diurnal variability in seawater oxygen as a response to the fluctuations in coral photosynthesis. Our study shows that coral physiology and surrounding seawater are inseparable. Physical, biochemical and biological processes at various scales should be analysed with an integrated approach. With the dynamic representation of two-way interaction between the corals and the surrounding seawater, our study improves our understanding of the coral reef ecosystems.